It is planned to use a library of cDNA clones from mouse kidney RNA to identify genetic variation in the DNA sequence of the inbred mouse strains. The variants will be recognized by hybridizing labeled cloned plasmid DNA's containing cDNA inserts to restricted, electrophoresed DNA from mouse livers which has been transferred to nitrocellulose filters or to DMB-paper. The source of DNA will be the seven progenitor strains of five sets of recombinant inbred (RI) strains. Each probe will hybridize to a set of restriction fragments with characteristic molecular weight, which can be seen by autoradiography of the nitrocellulose filters. If no variant exists each strain will show the same set of molecular weight fragments for a given probe. If a recognizable variant exists, the molecular weight of one or more of the restriction fragments will be altered. Such variants will be analyzed genetically using the appropriate set of recombinant inbred (RI) strains. Further mapping may involve use of appropriate congenic strains and progeny of crosses between appropriate parental strains. The use of recombinant inbred strains as a mapping tool currently gives a 40% chance of immediately mapping a new locus, and all loci will eventually receive map positions. Once the variants in restriction fragment length are mapped they will become important reference loci for mapping other genes. Analysis of 50 cDNA clones per year is envisioned. Many of these will detect variation. We were able to detect a variant with the first clone tested and thus project a rapid proliferation of new variants using this method.